1. Field of the Invention
This invention relates to the separation of isotopes. In particular, the invention relates to the separation of isotopes by pulsed electrolysis using the mass isotope effect and the magnetic isotope effect.
2. Description of Related Art
Isotope separation may be used to isolate a single isotope, or may be used to enrich a mixture of isotopes with respect to one or more isotopes in the mixture. The difference in physicochemical behavior between isotopes allows a variety of processes to be used for their separation. Commercially, the most important application of isotope separation is the enrichment of uranium. In order to be useful as a fuel for nuclear reactors, uranium must typically be enriched in 235U from a natural concentration of 0.7% to a concentration greater than about 2.5%.
Due to the relatively small difference in mass between 235U and 238U the enrichment of uranium is a difficult process. Although many processes have been demonstrated (e.g., laser separation) commercially viable enrichment is largely limited to centrifuge and diffusion techniques that employ uranium hexafluoride vapor as the working material.
Historically, gaseous diffusion has been the dominant process for uranium enrichment; however, the process is energy intensive and the gas centrifuge technique has been developed as a more energy efficient alternative. Although more efficient, centrifuges have the disadvantage of having a limited lifespan due to the high operational stresses to which they are exposed. Both diffusion and centrifuge techniques rely on the mass isotope effect.
Experimentally, the magnetic isotope effect has been shown to have potential as a basis for a separation process; however, with respect to uranium, commercialization has not been achieved. For many elements, the magnetic isotope effect has a greater potential for isotope separation than the mass isotope effect; however, problems such as isotope scrambling due to isotope exchange reactions have not been overcome. In order to minimize the deleterious effects of isotope exchange reactions, it is desirable to extract the products as soon as possible after formation. The prior art typically does not provide for rapid product extraction (e.g., less than one second).
Among the prior art techniques for isotope separation, one of the oldest is direct current electrolysis. Johnson and Hutchison, Urey, and others have demonstrated electrolytic reduction of lithium at a mercury cathode in both aqueous and non-aqueous electrolytes. More recently, isotope fractionation of iron has been disclosed by Kavner et al.
The boundary between a liquid electrolyte and an electrode typically has an associated region in the electrolyte adjacent to the electrode that separates the electrode from the bulk electrolyte. This region is often referred to as an interphase. Because of the influence of the electrode surface, the interphase has a composition that is different from the bulk electrolyte. The orientation of molecular dipoles and the concentrations of cationic and anionic species typically differ from the bulk.
Prior Art FIG. 1A shows a representation 100 of an interface between a metal and an electrolyte solution. Such an interface generally becomes electrified, with a net charge developing at the surface of the metal and a near surface region of excess ion concentration developing in the electrolyte solution. In the example of FIG. 1A, the excess charge in the metal is negative and the near surface region of the electrolyte solution has an excess concentration of cations.
Prior Art FIG. 1B shows a diagram 101 of the potential that is developed by the charge separation at interface between the metal and the electrolyte solution. The near surface region of the electrolyte solution is characterized by a “double layer” that is composed of a compact layer and a diffuse layer. The compact layer (or Helmholtz layer) is a thin region adjacent to the surface that typically contains adsorbed ions and oriented dipoles. The Helmholtz layer is also often further divided into two layers defined by an inner Helmholtz plane (IHP) and an outer Helmholtz plane (OHP). A discussion on electrode/electrolyte interfaces and the interphase is presented in “Modern Electrochemistry, Vol. 2A, Fundamentals of Electrodics, Second Edition,” by Bockris et al., Klewer Academic/Plenum Publishers (2000).
The Helmholtz layer typically has a thickness that is on the order of a nanometer. The diffuse layer has a less well-defined thickness that is frequently characterized by the Debye length (LD). For a 1:1 electrolyte the Debye length (LD) is given as:
      L    D    =                    1        ze            ⁢                                                  ɛ              r                        ⁢                          ɛ              0                        ⁢            kT                                2            ⁢                          c              0                                            =                            6.3          ×                      10                          -              11                                      z            ⁢                                                  ɛ              r                        ⁢            T                                c            0                              
T, z, e, εr, ε0, k and c0 are the temperature (Kelvin), valence number, electron charge, solvent relative permittivity, permittivity of free space, Boltzmann constant, and bulk electrolyte concentration (moles/m3), respectively. For water with a relative dielectric constant taken as 78 at a temperature of 298 K, a copper sulfate electrolyte solution at a concentration of one mol/m3 the diffuse layer has a calculated Debye length of about 10 nm. Due to the variability of the dielectric constant of the solvent close to the electrode and other phenomena, the calculated Debye length is only approximate, but it serves to illustrate the fine scale of the interphase in an electrolytic cell.
For redox reactions to occur at an electrode surface in an electrolytic cell, the reactants and products must traverse the interphase. The rates of reaction and the nature of the reaction products are thus influenced by the state of the interphase. A particularly important feature of the interphase is that large electric fields can be developed by the application of an electric potential.
When an electric potential is applied to an electrolytic cell, the interphase will adjust to the applied potential through a variety of mechanisms. Contact adsorbed ions may become dislodged and or replaced by counterions, molecular dipoles may change orientation, and the concentration profiles of cations and anions may change. The interphase differs from the bulk electrolyte in that an electric field can have a relatively greater influence on mass transport than diffusion. Although the interphase has been studied to a considerable extent, precise manipulation of the interphase has not been adopted on a manufacturing scale.
The speed at which an ion in an electrolyte solution will travel when subjected to an electric field depends in part upon the characteristics of the ion, the solvent, and the intensity of the electric field. Concentration and other factors may also influence the speed at which an ion travels. Due to the extremely short distances associated with the interphase, the adjustments that occur in the interphase in response to an applied potential can occur in a very short period, on the order of a microsecond or less. Thus, a potential waveform applied to an electrolytic cell that is intended to control the makeup of the interphase should be capable of providing precise potential levels and fast transitions between potential levels.
Ideally, a system for controlling the interphase will be able to produce a square pulse at the electrode surface with minimal rise time, overshoot, fall time, and undershoot. For industrial applications, the square pulse should be able to retain its characteristics when applied to large area electrodes. In order to achieve such a waveform at the electrode surface, all circuit elements in the current path should be considered.
Prior Art FIG. 2 shows a general schematic 200 for an equivalent circuit of an electrolytic cell. The schematic shows the resistive and reactive components of an electrochemical cell and its connections to a power supply. Cshunt is the parasitic capacitance that exists between the connections to the two electrodes in the cell. Cshunt is generally very small in comparison to the double layer capacitance presented by the electrochemical cell, and is only of concern for systems with extremely small electrodes.
RC1 and RC2 are the resistances associated with the leads connecting the electrodes to the power supply. For industrial applications in which hundreds of amps may be used at low working voltages, the magnitude of RC1 and RC2 is a matter of concern. Efforts are typically made to minimize conductor length and to provide sufficient cross-sectional area for the anticipated load. Copper bus bars or cables are widely used.
LC1 and LC2 are the inductances associated with the connections between the power supply and the electrodes, and are largely ignored in equipment intended for use at DC or low frequency. Even in equipment that is intended for applications such as reverse pulse plating, inductance is ignored to a considerable extent.
For example, U.S. Pat. No. 6,224,721, “Electroplating Apparatus,” Nelson et al., issued May 1, 2001, discloses the use of a coaxial conductor as a means for reducing inductance in a portion of the electrical distribution system for a plating bath. The preferred conductor assembly disclosed by Nelson is a loose circular coaxial configuration in which a tape-wrapped inner cathode conductor is placed in an outer anode conductor. Although preferred, the inductance of the coaxial segment is still on the order of 100 nanohenries. Further, Nelson does not address the inductance of the electrochemical cell itself or the requirements for control of the interphase in the electrolytic cell.
LEL1 and LEL2 are the inductances associated with the electrodes that are in contact with the electrolyte. The electrode inductance in industrial electrolytic cells is largely ignored, with factors such as current distribution and areal configuration taking precedence. REL1 and REL2 are the resistances associated with the electrodes that are in contact with the electrolyte. Typically, REL1 and REL2 are small compared to the resistance of the bulk electrolyte (RBE). For non-metallic electrode materials such as carbon or ceramic, resistance may influence design for use with high-conductivity electrolytes.
CDL1 and CDL2 are the double-layer capacitances associated with the electrodes that are in contact with the electrolyte. CDL1 and CDL2 can be quite large, but are seldom a concern for low frequency or DC electrodeposition systems. Although CDL1 and CDL2 can be adjusted, electrode shape and electrolyte composition are usually determined by other factors, with CDL1 and CDL2 being tolerated as an inevitable nuisance. In contrast to electrodeposition systems, a large CDL1 and CDL2 may be designed into electrochemical energy storage systems.
ZF1 and ZF2 are faradaic impedances associated with the charge transfer involved in redox reactions at the electrode surfaces. ZF1 and ZF2 are nonlinear, and dependent upon the electrode potential, nature, and concentration of the reactive species. In some respects, a faradaic impedance resembles the behavior of a reverse-biased diode, with a redox reaction potential being analogous to a breakdown voltage.
LBE and RBE are the inductance and resistance of the bulk electrolyte, respectively. LBE is largely ignored in the design of electrolytic cells. The current distribution and nature of the charge carriers in an electrolyte volume can be altered to adjust LBE, but they are usually adjusted in light of other design considerations. It is generally desired that RBE have a low value to reduce ohmic losses, and electrolyte composition often takes RBE into account. For example, sulfuric acid may be added to copper sulfate plating baths to reduce RBE.
Systems for instrumentation and analysis typically use relatively small electrodes and thus handle relatively small currents. The switching of small currents does not produce large voltage transients and the compact size of instruments serves to provide an inherent limit on inductance. Analytical electrochemical systems have also shown a trend toward ultramicroelectrodes (UMEs) in order to avoid problems in dealing with double-layer capacitance. The prior art instrumentation approach of using miniaturization to deal with reactive circuit elements is of little use for systems that are to be scaled for manufacturing processes.
During direct current electrolytic isotope separation, equilibrium conditions are established in the interphase in a very short time, and a natural consequence of a high reaction rate for one isotope at the electrode surface is a relative increase in concentration at the surface for the slower-reacting isotope. This increase in relative concentration limits the separation factor that can be achieved under direct current conditions.
In general, stirring of the bulk is not effective for disturbing the electrolyte layer adjacent to the electrode surface, and although hydrogen evolution is capable of producing local stirring, it has disadvantages. In order to achieve an enhanced electrolytic isotope separation factor, the interphase must be modified in a controlled fashion so that the limiting effect of preferred species depletion can be avoided.
Thus, there is a need for an electrolytic system and method that will provide for the selective oxidation/reduction of isotopes. There is also a need for an electrolytic system that is capable of employing the magnetic isotope effect and providing for rapid product extraction to minimize isotope scrambling due to exchange reactions.